A multilayer ceramic electronic component (also referred to hereinbelow as “multilayer electronic component”) is usually manufactured in the manner as follows. A ceramic raw material powder, such as a dielectric, magnetic, or piezoelectric material powder, is dispersed in a resin binder and formed into sheets to prepare ceramic green sheets (referred to hereinbelow as “ceramic sheets”). A conductor paste for an internal electrode that is prepared by dispersing an inorganic powder comprising an electrically conductive powder as the main component and, optionally, a ceramic powder or the like in a vehicle comprising a resin binder and a solvent is printed according to a predetermined pattern on the ceramic sheets and dried to remove the solvent and form dry films of the inner electrodes. A plurality of ceramic sheets each having the dry film of the inner electrode that were thus obtained are laminated and pressurized to obtain a non-fired laminate in which the ceramic sheets and paste layers of inner electrodes are alternately laminated. The laminate is cut to a predetermined shape, then subjected to a binder removal process in which the binder is thermally decomposed and dissipated, and fired at a high temperature whereby sintering of the ceramic layers and formation of the inner electrode layers are conducted simultaneously and a ceramic body is obtained. Terminal electrodes are then fired to both end surfaces of the body and a multilayer electronic component is obtained. The terminal electrodes and the unfired multilayer body are sometimes co-fired.
In recent years powders of base metals such as nickel and copper are mainly used instead of powders of noble metals such as palladium and silver as electrically conductive powders of conductor pastes for inner electrodes. Accordingly, firing of the laminate is usually carried out in a non-oxidizing atmosphere with an extremely low partial pressure of oxygen in order to prevent the oxidation of the base metals during firing.
There has been in recent years an ongoing trend towards smaller multilayer electronic components having higher layer counts. In particular, ceramic layers and internal electrode layers are becoming ever thinner in multilayer ceramic capacitors using nickel as a conductive powder.
However, the firing temperature of the capacitor is ordinarily 1200° C. or higher, which may give rise to oversintering of the nickel powder in internal electrodes. This oversintering causes various problems such as large voids after firing that result in an increase in resistance and greater apparent electrode thickness through spheroidization of electrodes brought about by excessive particle growth. These problems impose limits as to how thin the internal electrodes can be.
To render the electrodes thinner, conductor pastes for internal electrodes have come to use extremely fine nickel powders having a particle size of no greater than 1 μm, and even no greater than 0.5 μm. Such fine nickel powders have a high activity and a very low sintering initiation temperature. This leads to a disruption of the internal electrodes, since sintering starts at an early stage of firing. Specifically, when nickel particles are fired in a non-oxidizing atmosphere, even single-crystal particles with comparatively low activity begin to sinter and shrink at a low temperature of 400° C. or lower. By contrast, the temperature at which the ceramic particles comprised in the ceramic sheet begin to sinter is generally much higher than this. When co-fired together with the internal electrode paste comprising the above nickel powder, the ceramic layers fail to shrink together with the nickel films, as a result of which the nickel films are pulled in the planar direction. The small voids generated thereby in the nickel film, through sintering at a comparatively low temperature, are believed to expand into large voids as sintering progresses at a high temperature range. Large voids forming thus in the internal electrodes may give rise to higher resistance or circuit disruption, and may lower capacitance in a capacitor.
Moreover, the sintering shrinkage behavior of the internal electrodes and the ceramic layers may fail to be matched owing to volume expansion and shrinkage brought about by oxidation and reduction reactions of nickel during firing. This mismatch gives rise to structural defects such as delamination and cracks, and detracts from yields and reliability.
Moreover, fine nickel powders have a high surface activity. Therefore, when binder removal is carried out in a non-oxidizing atmosphere such as a nitrogen atmosphere, the nickel powder acts as a decomposition catalyst on the vehicle, which may cause the resin to decompose explosively at a temperature lower than its ordinary decomposition temperature. In such cases, the sudden gas release gives rise to cracks and delamination. Also, the suddenness of the reaction prevents the resin from dissipating completely and, as a result, there remains a carbonaceous residue. This is believed to be behind such problems as deterioration of capacitor properties, occurrence of structural defects, and loss of reliability. Specifically, when the residual carbon remaining in the internal electrode layers after binder removal is oxidized, gasified and dissipated during the subsequent sintering step of the ceramic at high temperature, it draws oxygen from the ceramic layers, thereby lowering the strength of the ceramic body and worsening electric properties such as capacitance, insulation resistance and the like. Carbon may also give rise to oversintering by lowering the melting point of the nickel powder.
With a view to solving these problems, for instance, Patent Document 1 discloses forming a dense oxide film, of a certain thickness, on the surface of a nickel powder, to minimize thereby the volume and weight changes caused due to the oxidation and reduction of nickel during firing and to raise the sintering initiation temperature, thereby preventing delamination. Although forming an oxide film on the surface of a nickel powder is effective in preventing structural defects and increases in resistance, virtually no effect is elicited thereby as regards to suppressing nickel oversintering. Moreover, although the oxide film present on the surface of the nickel powder is thought to have the effect of lowering the activity of the nickel surface, such activity increases steadily when particles are of a submicron size, in particular, of a size of 0.5 μm or smaller, and thus the above oxide film fails to suppress electrode discontinuity or deterioration of the properties caused by residual carbon during binder removal.
Patent Document 2, for instance, discloses adjusting the sintering temperature, and preventing the occurrence of delamination and cracks, by using a nickel ultrafine powder, of a specific particle size, containing 0.5 to 5.0% by weight of silicon. Also, Patent Document 3 discloses shifting the temperature of abrupt thermal shrinkage initiation to a higher temperature, and preventing structural defects such as delamination and cracks, by using a composite nickel fine powder in which an oxide such as titanium oxide, silicon oxide or the like is present on the surface of a nickel powder that is subjected to a surface oxidation treatment. This method, however, was also not effective enough for electrode thinning.
Patent Document 4 discloses forming internal electrodes of a multilayer ceramic capacitor by using a conductor paste containing, as a conductive powder, an alloy powder having an average particle size of 0.01 to 1.0 μm and comprising nickel as a main component, and no more than 20 mol % of at least one element from among ruthenium, rhodium, rhenium and platinum having a melting point higher than that of nickel, to thereby curb particle growth of the nickel powder at a firing stage, even when the internal electrode layers become thinner. Spheroidizing, circuit disruption, cracks and the like can be prevented as a result, and drops in capacitance are effectively curtailed. Patent Document 5 discloses that the same effect can be achieved in a conductor paste using a powder having a coating layer that comprises at least one element from among ruthenium, rhodium, rhenium and platinum, on the surface of a nickel powder.
Patent Document 1: Japanese Patent Publication 2000-45001 A
Patent Document 2: Japanese Patent Publication 11-189802 A
Patent Document 3: Japanese Patent Publication 11-343501 A
Patent Document 4: WO 2004/070748
Patent Document 5: Japanese Patent Publication 2004-319435 A